Lead dioxide electrode

ABSTRACT

Disclosed is a novel lead dioxide electrode excellent in shock-resistance, chemical-resistance and electrical conductivity, which has as the electrode proper at least one set of double layer consisting of an α-lead dioxide layer and a β-lead dioxide layer.

BACKGROUND OF THE INVENTION

This invention relates to a novel lead dioxide electrode excellent inshock-resistance, corrosion-resistance and electrical conductivity andfree from electrodeposition strain.

Conventional lead dioxide electrodes are used as electrodes in theelectrolytic oxidation for the manufacture of halogenates. Efforts arenow being continued to develop applications for lead dioxide electrodesto be used as electrodes in the electrolytic treatment of waste water oras anodes in the diaphragm-process electrolysis of sodium chloride.

The manufacture of lead dioxide electrodes has heretofore been carriedout by an acidic electrodeposition process which uses lead nitrate, forexample, as the electrolyte. This process causes lead dioxide to beelectrodeposited on the substrate. The lead dioxide layer consequentlyformed on the substrate consists preponderantly of β-PbO₂. The layer,therefore, inevitably suffers electrodeposition strain, entailing thedisadvantage that the layer itself may develop cracks or may break whenthe formed layer is peeled off the substrate.

For the purpose of obtaining an electrode having fastness high enough towithstand the electrodeposition stress productive of internal strain,attempts have been made to improve the shape of the substrate, thecomposition of the electrolyte, and various other electrolyticconditions including use of additives. Perfect elimination of theelectrodeposition strain from the β-PbO₂ layer obtained by theelectrolysis in an acidic bath is impossible. The electrodes of the typeformed of such layer, therefore, are deficient in shock-resistance andleave much to be desired from the practical point of view.

An object of the present invention is to provide a lead dioxideelectrode which is practically free from electrodeposition strain and isexcellent in electrical conductivity, corrosion-resistance,chemical-resistance and shock-resistance.

Another object of this invention is to provide a lead dioxide electrodethe manufacture of which is very easy.

SUMMARY OF THE INVENTION

To accomplish the objects described above, the lead dioxide electrodeaccording to this invention has as the electrode proper at least one setof double layer which consists of an α-lead dioxide layer and a β-leaddioxide layer.

The α-lead dioxide layer, when electrodeposited under specificconditions, does not permit development of electrodeposition strain. Byallowing the β-lead dioxide layer which excels in corrosion-resistanceand electrical conductivity to be joined fast to said α-lead dioxide,the electrodeposition strain possessed inherently by the β-lead dioxidelayer is alleviated to such an extent that there is consequentlyobtained a lead dioxide electrode which is practically free fromelectrodeposition strain and is excellent in electrical conductivity andcorrosion-resistance.

Furthermore, the process merely comprises the steps of electrodepositingan α-lead dioxide layer and a β-lead dioxide layer in the ordermentioned on a substrate by an ordinary technique of electrolysis andremoving the substrate from the formed layer as occasion demands. Themanufacture of the electrode of this invention is easy because it doesnot involve complicated steps of the conventional technique such as incausing a layer of lead dioxide to be electrodeposited on the inner wallsurface of an iron cylinder and cutting segments of the layer off thewall surface.

The other objects and characteristics of the present invention willbecome apparent from the following detailed description of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It has been an accepted belief that electrodeposition strain never failsto occur and persist in the lead dioxide layer and defies all attemptsat elimination. The inventor made a devoted study on electrodepositionstrain, electrical conductivity, corrosion-resistance, etc. with respectto α-PbO₂ and β-PbO₂ layers. He has consequently acquired a knowledgethat the α-PbO₂ layer, though slightly inferior to the β-PbO₂ layer incorrosion resistance and electrical conductivity, excels in ability toadhere to the substrate, and that internal stress in the α-PbO₂ layercauses the α-PbO₂ layer to change its state continuously from tensilestate (an outwardly bowed state wherein the strain is expansive) tocompressive state (an inwardly bowed state wherein the strain iscontractive) by proper selection of the electrolytic conditions,therefore, it is possible to find out certain sets of conditions of thevariables under which no internal stress--namely no electrodepositionstrain (an intermediary state wherein there is a complete absence ofstrain) --develops, and that, on the otherhand, the β-PbO₂ layer excelsin corrosion resistance and electrical conductivity and enjoys highefficiency of manufacture but fails to enjoy freedom fromelectrodeposition strain. It has been ascertained that an electrodewhich suffers little from electrodeposition strain, exhibitsshock-resistance of a sufficient degree from the pratical point of viewand, what is more, electrical conductivity, corrosion-resistance andchemical-resistance which are important attributes for electrolysis canbe obtained by causing these two layers to be electrodeposited one ontop of the other in the form of an electrode proper so as to make themost of the characteristics inherent to the two layers. The presentinvention has been completed on the basis of this knowledge.

To be specific, this invention relates to a lead dioxide electrode whichis comprised of at least one set of double layer consisting of an α-PbO₂layer and a β-PbO₂ layer formed on a substrate or of said set of doublelayer minus said substrate.

In each set of double layer in the electrode of the present invention,the α-PbO₂ layer is desired to have a thickness of not less than 0.1 mmand the β-PbO₂ layer to have a thickness in the range between 0.2 and1.0 mm. If the thickness of the α-PbO₂ layer is less than the lowerlimit 0.1 mm, then the electrodeposition strain which develops in theβ-PbO₂ layer cannot be completely alleviated and the pinholes which tendto occur in the α-PbO₂ layer cannot be thoroughly eliminated. In thiscase, the characteristic properties of the electrode as a whole arescarecely improved and the phenomenon of electrodeposition strain aloneis aggravated when the thickness of the β-PbO₂ layer is increased tomore than 1.0 mm for the purpose of compensation. The combined thicknessof one set of double layer is desired to be not less than 0.3 mm. Theelectrode fails to exhibit sufficient fastness if the combined thicknessis less than 0.3 mm.

The exposed surface or active surface of the electrode may be that ofeither the α-PbO₂ layer or the β-PbO₂ layer. In consideration of thefact that the β-PbO₂ layer excels the α-PbO₂ layer incorrosion-resistance and electrical conductivity and the α-PbO₂ layerexhibits a better ability to adhere to the substrate than the β-PbO₂layer, it may be more practical to use the active surface of the β-PbO₂layer.

The aforementioned double layer may be used in a state deposited on thesubstrate or it may be used as a complete electrode in a state strippedof the substrate. In either case, the electrode functions on entirelythe same operating principle. It serves as an electrode which sufferslittle from electrodeposition strain and excels in corrosion-resistanceand electrical conductivity.

The substrate to be used with the electrode of the present invention isnot specifically limited. It is only desired to be of a substance suchthat it enjoys insolubility, electrical conductivity, light weight andample fastness and exhibits an expansion coefficient approximating thatof lead dioxide. Examples of substances which satisfy these requirementsand which are inexpensive include graphite, titanium, iron and stainlesssteel.

The method by which the electrode of the present invention ismanufactured will be described in full detail below.

Preparatory to the electrodeposition, the substrate is desired toundergo a pretreatment such as for removal of grease or rust. Where theα-PbO₂ layer is first electrodeposited, such pretreatment may be omittedbecause the electrolyte is alkaline and consequently theelectrodeposited layer exhibits satisfactory adhesiveness. Where theelectrical conductivity is particularly required, the substrate isdesired to undergo said pretreatment followed by a treatment for silverplating.

The conditions for electrodepositing α-PbO₂ layer of the type free fromthe electrodeposition strain depend on the combination of such factorsas the composition and concentration of the electrolyte and the densityof positive electric current. Generally, the electrodeposition iscarried out by passing an electric current in an electrolyte having alead concentration of from 0.1 to 0.5 mol/liter and an alkaliconcentration of from 3 to 10N at a temperature in the range of fromroom temperature to 80° C, with the density of the positive currentcontrolled in the range of from 1 to 5 A/dm². With the start of thepassage of said electric current, the active surface side of thesubstrate begins to be coated with a film of lead dioxide. Theelectrodeposition is continued until the coat thus formed increases to arequired thickness. At the end of the electrodeposition, the layerdeposited on the substrate is washed with water and dried. The α-PbO₂layer thus formed on the substrate is free from electroposition strainand, therefore, may be safely dried by application of heat.

The electrodeposited layer thus obtained has a purplish black, partlyglossy compact texture and exhibits fast adhesiveness. If the layer isformed to a thickness of 0.3 mm or more, it could then be used as anelectrode complete in itself. If the thickness is so small as 0.1 to 0.2mm, however, the layer may possibly suffer from occurrence of pinholesand therefore cannot be used safely in its unmodified form. Such a smallthickness may suffice for this layer insofar as the β-PbO₂ layer isadditionally electrodeposited thereon.

Then, the β-PbO₂ layer is electrodeposited on the α-PbO₂ layer. Thiselectrodeposition is effected by effecting an acidic electrolysis usingas the electrolyte a concentrated solution of lead nitrate. To bespecific, the acidic electrolysis is carried out in an aqueous 25%Pb(NO₃) solution, for example, with the positive current density fixedin the range of from 5 to 10 A/dm² and the solution temperature held inthe range of from 50° to 60° C. The electrodeposition liquid is desiredto be used in a fluidic state. The spent liquid emanating from theelectrodeposition bath may desirably be received in a neutralizingvessel to be completely neutralized with lead carbonate or leadhydroxide, for example, and returned in the neutralized state back tothe electrodeposition bath for reuse.

In this electrolysis, the PbO₂ double layer of a thickness of the orderof 2 to 3 mm can sufficiently be obtained in a matter of two to threehours because the electrolyte used has a high concentration and thepositive current is used in a high density. The current efficiency forthe formation of the PbO₂ layer is on a relatively high level of 83 to85%. The electrodeposited β-PbO₂ layer has a purplish black color and asurface flecked with fine particles. Compact in texture, this layerenjoys a higher degree of fastness than the α-PbO₂ layer (Martens'scratch hardness -- 22 for β-PbO₂ layer and 20 for α-PbO₂ layer).

Where the electrode is desired to be manufactured in a sheet-like formcontaining no substrate, it can be obtained by first electrodepositingon one surface of the substrate an α-PbO₂ layer and thenelectrodepositing thereon a β-PbO₂ layer by following the proceduredescribed above, subsequently repeating this cycles of operation to haveadditional α-PbO₂ layers and β-PbO₂ layers electrodeposited alternatelyuntil the combined thickness of layer reaches a required value (about 10mm), and thereafter separating the substrate mechanically by use of acutter or, if the substrate happens to be made of iron, chemicallydissolving out the substrate from the substrate by use of an acid.

In this case, a plate-shaped electrode which has a β-PbO₂ layer oneither surface thereof can be obtained by carrying out theelectrodeposition of alternating layers in such way that the first andlast layers are both of β-PbO₂.

As described above, the electrode of the present invention is given atleast one set of double layer consisting of an α-PbO₂ layer and a β-PbO₂layer by causing α-PbO₂ layers and β-PbO₂ layers to be alternatelyelectrodeposited one on top of the other. In the electrode thusproduced, the α-PbO₂ layer enjoys good adhesiveness to the substrate andfreedom from electrodeposition strain. Moreover, since an alkalielectrolyte is used for the electrodeposition of the α-PbO₂ layer, therestrictions which would be imposed in the case of the acidicelectrolysis on the selection of materials of substrate, electrolyticcell, etc. are substantially removed.

In the case of an electrode having the active surface (outermost layer)of β-PbO₂, since the β-PbO₂ layer excels the α-PbO₂ layer in terms ofcorrosion-resistance and exhibits high electrical conductivity and hasits inherent weak point of electrodeposition strain alleviated to someextent by the α-PbO₂ layer, the electrode is notably improved in itscharacteristics in electrolysis so as to materialize savings of bothproduction time and cost.

Lead dioxide electrodes have always drawn particular attention for theirspecific performances as anodes in the manufacture of hydrohalogen acidsalts. Recently, they have been expected to find extensive utility inelectrolysis of sodium chloride and in electrolytic disposal of wastewater as well.

The present invention makes possible the manufacture of a lead dioxideelectrode which suffers little from internal strain, exhibits notablyimproved fastness, electrical conductivity and corrosion-resistance andenjoys light weight, low cost and high practical utility. In addition tothe uses mentioned above, the electrode of this invention is expected tofind new applications such as in electrolytic metal refining,electrolytic floatation, electrolytic dialysis, etc.

Now the present invention will be described with reference to examples,which are cited solely for illustration and should be considered aslimitations of the invention.

EXAMPLE 1

An electrolyte prepared by dissolving 80 g of lead hydroxide in 2 litersof an aqueous 5N caustic soda solution was placed in an electrolyticcell. In the electrolytic cell, a titanium electrode measuring 50 mm inlength, 20 mm in width and 0.3 mm in thickness, as the anode, and twostainless steel sheets having dimensions identical with those of theanode, as the cathodes, were disposed at fixed intervals of 50 mm. Withthe electrolytic cell, the electrolysis was carried out for three hours,with the amperage fixed at 500 mA, the electrolytic bath temperature at50° C and the bath voltage at 2.5 V respectively. The current efficiencywas nearly 100%. After the electrolysis, the anode was washed with waterto be freed completely from the alkali and measured for thickness. Themeasurement showed the thickness of the formed α-PbO₂ layer to be 0.2mm. On this anode, such phenomena as deformation due to inner strain andexfoliation of the formed α-PbO₂ layer were not observed at all.

Subsequently, the electrode on which said α-PbO₂ layer had been formedwas used as the anode and two stainless steel sheets having the samedimensions were used as the cathodes. In an electrolytic cell containing5 liters of an aqueous 25% lead nitrate solution, said electrodes weredisposed. With this electrolytic cell, the electrolysis was carried outat a constant current for about 5 hours, with the anode current densityfixed at 2 A/dm² and the electrolytic bath temperature at 60° C. Thiselectrolysis was carried out by the reflux neutralization process, witha basic lead carbonate used as the neutralizing agent. The β-PbO₂ layerwhich had been electrodeposited on the surface of said anode had apurplish black color, a surface slightly flecked with fine particles anda thickness of 0.5 mm. The combined thickness of the PbO₂ double layerconsisting of the α-PbO₂ layer and the β-PbO₂ layer was about 0.7 mm.

In spite of such a small thickness, the PbO₂ layer did yield whatsoeverto slight impacts. It enjoyed unusually high fastness and perfectfreedom from discernible phenomena of cracks and exfoliation.

By way of performance test, the electrode thus obtained was subjected toelectrolytic oxidation using potassium perchlorate.

The electrolysis was carried out in the absence of a diaphragm for about10 hours by using a potassium chlorate solution with a concentration of5 mols/liter as the raw solution and a stainless steel sheet as thecathode, with the bath temperature fixed at 15° C and the anode currentdensity at 50 A/dm² respectively. The current efficiency for theformation of potassium perchlorate was found by this test to be about87%.

The results indicate that the current efficiency obtained by the presentelectrode is about 5% higher than that obtained in the electrolysiscarried out with the conventional plate-shape electrode composed mainlyof a β-PbO₂ layer, that the evolution of heat during the electrolysis ismuch smaller despite the higher current density and that the phenomenasuch as change in the active surface and decay of the electrode properwere not observed at all.

EXAMPLE 2

A titanium lath measuring 50 × 150 mm was used as the anode and twocopper sheets of the same size were used as the cathodes. In anelectrolytic cell, these electrodes were disposed at fixed intervals of20 mm. A liquid obtained by dissolving an excess amount of lead oxide in5 liters of an aqeous 4N caustic soda solution so that the solution wassupersaturated and the excess lead oxide sedimented at the bottom ofsolution was used as the electrolyte. In this electrolyte, an electriccurrent was passed for 2 hours, with the anode current density fixed at2.5 A/dm², the bath temperature at 40° C and the bath voltage at 1.2 Vrespectively. Consequently a rigid purplish black layer of lead dioxidewas deposited to a thickness of 0.2 mm on the anode lath. This layer,when examined by X-ray diffraction, was identified to be pure α-PbO₂.

On the α-PbO₂ layer, the passage of electric current was continued forhalf an hour under virtually the same electrolytic conditions as thoseemployed in the stage of β-PbO₂ layer production in Example 1 exceptthat the anode current density was changed to 10 A/dm². Consequentlythere was formed a β-PbO₂ layer having a thickness of about 0.2 mm.Although pinholes occurred to some extent in the α-PbO₂ layer, they didnot interfere with the electrodeposition of the β-PbO₂ layer at all.While the current efficiency was 100% during the formation of the α-PbO₂layer, it was about 85% during the production of the β-PbO₂ layer.

The anode thus produced by the electrodeposition of the α-PbO₂ layer andthe β-PbO₂ layer was washed with water, dried at 60° C, and thensubjected to electrodeposition first of an α-PbO₂ layer and then of aβ-PbO₂ layer over a total period of 5 hours. The thickness of the formedα-PbO₂ layer was 0.2 mm and that of the β-PbO₂ layer was 0.5 mm.

By the procedure described above, there was obtained a multi-layerelectrode which had two sets each of an α-PbO₂ layer and a β-PbO₂ layerand had a combined thickness of 1.1 mm.

In a two-compartment electrolytic cell having an asbestos diaphragm,batchwise electrolysis of sodium chloride was carried out by using thiselectrode. The electrolysis was tried under varying combinations ofconditions, with the anode current density selected from the range of 20to 50 A/dm² and the temperature from the range of 20° to 70° Crespectively. The results show that the current efficiency for theformation of caustic soda was from 85 to 90% and the wear of electrodefrom 0.06 to 0.5 gr/KAH in the catholyte. The values indicate that theelectrode is amply suitable for the practical use.

EXAMPLE 3

By using as the substrate a graphite plate measuring 200 mm in length,70 mm in width and 40 mm in thickness and by adopting the sameconditions as used in the electrolysis for the β-PbO₂ layer depositionin Example 1, a β-PbO₂ layer of a thickness of about 1 mm waselectrodeposited on one surface of said graphite substrate. On thisβ-PbO₂ layer, an α-PbO₂ layer having a thickness of 1 mm waselectrodeposited under the same conditions as those used in theelectrolysis for deposition of the α-PbO₂ layer in Example 1. The twostages of operation were repeated. The PbO₂ layer were examined toconfirm that the layers were deposited fast on the substrate and theywere free from possible cracks and exfoliation. Thereafter, a β-PbO₂layer 3 mm in thickness, an α-PbO₂ layer 2 mm in thickness and anotherβ-PbO₂ layer 3 mm in thickness were electrodeposited to afford anelectrode having a PbO₂ layer 12 mm in thickness electrodepositedthereon.

Then five grooves were cut in the graphite substrate by use of athin-blade grinder to separate the PbO₂ layer from the substrate.Consequently, there was obtained a thin electrode containing nosubstrate.

The electrode thus obtained was observed to have substantially nointernal strain. This electrode was not broken under slight impacts.

What is claimed is:
 1. A lead dioxide electrode having at least one setof double layers consisting of an α-lead dioxide layer of thickness notless than 0.1 mm and a β-lead dioxide layer of thickness not less than0.2 mm as the outer layer.
 2. A lead dioxide electrode according toclaim 1, wherein said set of double layers is deposited on a substrate.3. A lead dioxide electrode according to claim 2, wherein the α-leaddioxide layer is deposited on the substrate.
 4. A lead dioxide electrodeaccording to claim 1, wherein the β-lead dioxide layer has a thicknessin the range between 0.2 mm and 1.0 mm.
 5. A lead dioxide electrodehaving at least one set of double layers consisting of an α-lead dioxidelayer of thickness not less than 0.1 mm and a β-lead dioxide layer ofthickness not less than 0.2 mm and having the β-lead dioxide layers asthe outer face of each double layer.